1. Field of the Invention
The present invention relates to an apparatus and method for image forming, and more particularly to an apparatus and method for image forming capable of preventing toner offset in an image fixing by using a cooler.
2. Discussion of the Background
In a background image forming apparatus using Electrophotography such as a copier, a printer, and a facsimile, an image is formed through various steps including charging, exposing, developing, transferring, fixing, cleaning, and discharging.
In the fixing step, a toner image is generally fixed to a transfer sheet by heating and pressing the toner image using a fixing roller and a pressure roller. However, the heated toner may be melted and adhered to the surface of the fixing roller, which comes into direct contact with the toner. The toner adhered to the surface of the fixing roller may be then transferred back to the transfer sheet (the so-called “toner offset”), thereby deteriorating the image quality.
One approach for preventing toner offset is to use toner containing a polymer having a high molecular weight. However, the toner has such a high glass transition temperature that an image needs to be fixed at a high temperature. Thus, high-speed image forming or energy saving may not be easily attained.
To meet the demand for high-speed image forming or energy saving, toner containing a polyester resin having a low fixing temperature is preferably used. In practice, however, the toner containing a polyester resin tends to melt at a lower temperature, thereby causing the toner offset problem more easily. Therefore, there is a need for preventing toner offset, particularly when a toner containing a polyester resin is used.
In order to prevent toner offset, the temperature of the background image forming apparatus is controlled by stopping or slowing down its operation. Such a background image forming apparatus successfully prevents toner offset, however, it suffers from a lower productivity. In addition, the background image forming apparatus fails to completely eliminate the toner offset when a toner containing a polyester resin is used.